What lengt crankarms for SX trail?

Hi, I am currently building up an SX trail and I just bought 175 mm lenght crankarms. The guy at my shop says that they might be too long and that I'll be hitting rocks with my pedals a lot. What length do you think is right?

I run a 175mm Saint Double on my 07 SXTIII and havent had any issues with them. I do run the High BB shuttle on it. I use mine for most aggressive XC and Freeride, and do some park riding with it.

I used to run the low shuttle also, and didnt have an problems, but i like the high setup better for where i ride, its pretty tight stuff, if i was using it as strictly a DH rig, i would run the 170's, but for what i do, the 175's are great.